In the manufacture of fiber reinforced resin products, sheet molding compounds are frequently used. Sheet molding compounds offer an appealing solution for the production of Class A surface parts compared to steel both in terms of cost and coefficients of thermal expansion.
Sheet molding compounds consist of a mixture of a liquid thermosetting resin, particulate filler and chopped reinforcement fibers, such as glass fibers. In most cases, the resin and chopped fibers are sandwiched between films of plastic material to form a laminated sheet that is wound in rolled form or festooned for storage. The laminated sheet is stored under conditions that will not result in final curing of the resin, but will allow the paste to thicken to a desired molding viscosity range, typically between 30,000 and 50,000 centipoise (MilliPascal seconds). At the time of use, the protective carrier film is removed and the laminated sheet is cut into blanks, or plies, of a desired shape and size. The plies are then molded to form a cured composite part. In most applications, multiple plies of the laminated sheets are used in the composite structure and typically comprise between 25 and 50% of the die/tool's surface area. When the laminated sheets are molded, the resin and glass flow within the mold under heat and pressure to cover the entire surface of the mold. Sheet molding compounds are used in a variety of applications that require aesthetic appeal, corrosion resistance, lighter weight dimensional control and high strength.
One deficiency with currently available sheet molding compounds is that the charge typically does not form a Class A type surface parts when cured. This is due to the fact that the chopped fibers move to the surface of the sheet molding compound to form surface imperfections. Further, the fiber used in some sheet molding compounds typically does not flow well in the mold, and this creates surface imperfections such as surface pores. Thus, sheet molding compounds require sanding and polishing, or otherwise reworking to be used in applications requiring a desired surface appearance.
Yet another problem with surface characteristics occurs when these composite parts formed from the sheet molding plies are painted. Paint pops may be caused by the release of volatile liquids (such as water, styrene or di-vinyl benzene monomer) from the sheet molding paste or by the release of moisture or solvents contained within fiber bundles during the curing process are quite common, typically affecting 5–10% or more of painted SMC composite parts. This leads to substantial cost in terms of rework and waste.
In addition, to allow a long and uniform flow that will produce a wavefree surface, the fibers used in sheet molding compounds are typically provided by the glass manufacturer as bundles or “splits” of multiple filaments. The act of impregnating the bed of chopped fibers between two layers of sheet molding compound paste often leaves air trapped within the composite sheet, most often besides the bundles where small differences in surface tension adversely affects the wetting of the bundles or splits. Unfortunately, this bundling may also include entrapped air which, when released during the flow, produces tiny bubbles which travel slowly under a pressure gradient. To evacuate these bubbles, it is useful to have the molding compound flow to fill out the tool to allow the action of the pressure gradients to move those air bubbles towards the edge of the flow front and thus towards the edge of the part. Such large flow typically calls for loading the tool by a charge representing 50% or less of the area of the part
It is therefore highly desirable to improve the surface characteristics of sheet molding compound. This would allow sheet molding compound parts to be used in a wider variety of composite applications wherein surface quality is a concern.